Support finding compile_commands.json in non root project directories

[ChaseKnowlden]

Before: focus: undefined

After:

19:13:49: Activating Swift for Visual Studio Code...
19:13:50: SourceKit-LSP setup
19:13:51: swift-data: add: /Users/chase/dev/swift-data
19:13:51: swift-data: focus: /Users/chase/dev/swift-data

Fix #1449

[plemarquand]

I think we should do the same as sourcekit-lsp and search in . and build, but hold off doing an expensive recursive search of all folders in the workspace folder.

To solve this properly I think we'd need to implement a setting in sourcekit-lsp to search for compile_commands.json in a supplied set of folders, and then mirror that setting in the VS Code extension, passing it to sourcekit-lsp on startup and using it here in isValidWorkspaceFolder as somewhere to search.